Fabry-Perots are well-known and important optical devices. They may be used as a true interferometer, (e.g., for spectroscopy), as a laser cavity or, more particularly, as an insert in the laser cavity of a single mode laser. Currently, Fabry-Perots are also being suggested as reflection or transmission modulators for optical interconnects.
A Fabry-Perot commonly comprises two reflective surfaces the "mirrors") separated by a determined optical path length (the "gap"). In a Fabry-Perot interferometer the gap enclosed by the mirrors usually comprises air and can be mechanically varied, e.g. by moving one of the mirrors. In a Fabry-Perot etalon the mirrors are usually held fixed, for example by means of a spacer, for which, for example, quartz, or glass is commonly used. Common gap widths for an interferometer vary from several millimeters to several centimeters. Considerably greater gap widths are customary when a Fabry-Perot is employed as a laser resonant cavity. For some applications, including modulators, gap widths may be of the order of one to several micrometers.
In general, two types of mirrors are used in Fabry-Perots, namely, so-called quarter-wave stacks (QWS) of a material that would not exhibit substantial reflectance as a single layer, or mirrors comprising a single layer of a material that is in itself reflecting, usually comprising metal or metal compounds. The mirrors frequently are plane parallel, but curved-mirror systems are also known, notably as laser cavities and as spectrum analyzers.
The invention pertains to Fabry-Perots of the type comprising two mirrors of a reflecting material, such as metal. As is apparent from the above, the inner surfaces of the mirrors are separated from each other to form the gap, the gap width optionally being fixed.
British Patent No. 2,082,380 discloses a Fabry-Perot etalon of this type used in a semiconductor injection laser. The Fabry-Perot is provided with a quarter-wavelength anti-reflection layer on the inner surface of one of the mirrors in order to broaden the spectral emission, to increase the range of single mode operation, and to reduce spread on the far-field pattern. The inner surface of the other mirror is optionally coated with a half-wavelength layer.
In Annales de Physique 6 (1951), pages 5 and following, Fabry-Perots are disclosed, the metal mirrors of which are provided with a dielectric layer on the side facing the gap. In the disclosure it is concluded that a single dielectric layer is unsuitable to significantly improve the reflectance of a Fabry-Perot.
European Patent Publication No. 371,695 pertains to a spatial light modulator on the basis of a QWS Fabry-Perot in which the gap comprises a liquid crystal layer and which Fabry-Perot includes metal electrodes. The mirrors in this device are dielectric multilayer films, namely, quarter wave stacks.
QWS Fabry-Perots have also been disclosed in such representative references as Thin solid Films vol. 137, No. 2, pages 161-168 (1968) and Optik, Vol. 50, No. 4, pages 329-340 (1978).
The state of the art further includes the common knowledge which exists in regard to Fabry-Perots. The original interferometer of this type developed by C. Fabry and A. Perot, for instance, comprises two transparent (glass or quartz) plates with planar surfaces. The inner surfaces are coated with partially transparent films of high reflectivity and are parallel, enclosing an air gap. The transparent plates serve as a substrate for the reflecting coating. These and other known Fabry-Perot mirrors commonly comprise a metal, such as gold, silver or aluminum, or a metal compound, as a reflecting surface.
Naturally, the mirrors of a Fabry-Perot exhibit substantial reflectance, e.g. higher than 90%, but to a certain extent they also show transmittance. Further, mirrors comprised of a material that is in itself reflecting, notably metal mirrors, display absorption of incident light.
The mirror reflectivity being high, the reflectance and transmittance show a series of peaks as a function of the wavelength of the incident light. If the mirror separation is much larger than the wavelength (which is usually the case) the peaks are approximately periodical in the wavelength. The ratio of the distance separating subsequent peaks (the free spectral range, or FSR) to the full-width at half maximum of the peaks is frequently used to specify the quality of a Fabry-Perot. Said ratio is a dimensionless quantity called the "finesse". For many applications a high finesse is desirable.
In order to achieve a high finesse, a high reflectance is generally required. In the case of Fabry-Perots of the type having mirrors comprising metal or metal compounds, increased reflectance usually is attended with increased absorbance. Consequently, in Fabry-Perots of the present type the demand of high finesse competes with that of high peak transmittance. It is therefore desired to improve Fabry-Perots with respect to peak transmittance, without the finesse being adversely affected.